Hi

I'm looking for some help with regards to a tach problem.
I have a 92 Seville 4.9L with analog cluster. The tach has gone wacky in that it reads at least 1000rpm more at idle and says the shift points during normal accel. are at 6500-7500rpm.
The cluster unit is hard to find and probably very pricey from dealer so maybe I can troubleshoot it as it might not be the tach at all. Does the unit get it's signal from a wire connected to the coil? Could it be a faulty ground or is it done?
It almost reacts like a 4 or 6 cyl tach connected to a v-8.

Any help appreciated

TIA
Frank

If you take a tach from a four or six cylender engine and put it on a V8 it would not read any different than the original V8 tach. Remember its only measuring how many rotations the engine makes per minute. Of course an engine with balanced cams and a balanced crankshaft can reach higher RPMs, but that is the only way it can reach those higher RPMs without vibrating itself to pieces. It may be the sensor going bad and giving bad information to the tach, but it could be the tach itself also. I would suggest that you check the codes. Since this is an electrical problem it should give you a code to point you in the right direction.

Ohh yeah almost forgot. Welcome!! :welcome:

Thanks for the reply and the welcome note.
I think that maybe I was misunderstood. The tach is the original Cadillac unit and not an aftermarket one, anyway I have checked the codes and none come up. After doing some research on the net I have found out that on the module inside the distributor there is a wire (purple) that controls the tach signal and also there is a wire from the dist. marked "Tach". I removed the wire and started the car and noticed that the tach moved to 5000rpm and stayed there, no movement at all. I put the wire back and the unit continued to act funny. I don't really want to change the module as it's just a guess that it might be faulty. I guess I'm trying to eliminate all/most of the things that it could be before dealing with the guage cluster.
Can a module cause these kind of problems or is it a thing that if something goes the car would not start?

Well judging by what you just said, I would suspect that you need a new distributor cap, and maybe rotor. In my last post, my point was that if you were to hook up any tach to your engine it should read exactly the same as the original. How many cylenders an engine has doesn't effect the tach reading. I don't think your tach is broken, I think your sensor is going bad in the distributor cap.

thanks again for your help. I will be changing the cap and rotor this weekend and see if that changes anything. I will repost letting you know if it solved the problem.

Thanks again for the help